* pinentry/pinentry.c (pinentry_utf8_to_local): Declare INPUT
const.
(pinentry_local_to_utf8): Likewise.
(pinentry_utf8_to_local, pinentry_local_to_utf8): Compile only, if
either Curses or GTK+ support is enabled.
* configure.ac: Do also check for libiconv when the GTK+ version
of pinentry is to be build. Define PINENTRY_CURSES, PINENTRY_GTK,
PINENTRY_QT depending on which versions of pinentry should be
build.